Most tanks in chemical plants, refineries, and similar factories are custom designed vessels that have to be cleaned periodically. Since the tanks are custom designed and thus may have different interior geometries, no one cleaning system will work adequately for all tanks. Furthermore, vessels are typically divided with dividing plates which include centered through-holes or partially removable dividing plates. Also, many processes in these types of plants or factories leave a hard, tenacious scale on interior surfaces of tanks, which presents an especially difficult cleaning problem.
Commonly, such a tank has an entry point or access way which is small relative to the interior diameter and height of the tank. On the other hand, a typical tank has relatively large inner surface areas which require periodic cleaning to remove the buildup of materials left by the material kept in the tank, such as calcium and magnesium carbonates and similar residues. Thus, a single manufacturing facility may have a wide variety of tanks of varying sizes, each requiring this sort of periodic maintenance and at least some of the tanks presenting a different aspect of interior geometry versus the size of the entry point or access way.
That restriction presents the engineering dilemma of having to insert the tool through a small opening (so the tool has to be small), but requiring a substantial distance for a water jet from the tool, in order to reach the farthest surfaces of the interior of the tank. To remove hard scale from the interior surfaces of a tank, the water jet must be operated at a high pressure, for example at least 9,000 psi, and the jet must be positioned in close proximity to the tank wall surface, for example at six inches or closer, in order to be effective.
With all of these factors in mind, one can see that it is difficult to find a single cleaning tool that fits all tank sizes and applications while doing a good job of cleaning the interiors of all of the tanks. One current proposed solution available on the market uses a small volume, high pressure water cleaning tool that is positioned inside the vessel and moved along the center axis of the tank while several water jets rotate around one or two axes simultaneously. Since the water jets are directed more or less radially from one point inside the tank, the distances from the water jet exit ports to the vessel walls are substantial and change continuously. For portions of the interior tank wall that are more than six inches from the water jet, hard scale is not removed and remains on the wall
For this type of water jet cleaning system, surface coverage cannot be exactly controlled since the water jet tracks contact the interior surface of the tank at more or less random locations. For proper surface coverage, each track of the water jet should overlap the previous track by a small amount. If the track does not overlap a previous track, then a portion of the interior surface of the tank will not be cleaned. If there is too great an overlap, then the track will be directed too much to a portion of the interior surface which has already been cleaned and the process is therefore inefficient.
However, in many known systems, the tracks of the jets are directed more or less randomly. That means that in order to insure that the entire interior surface of the tank is cleaned, the cleaning process must be continued for a much longer period of time than would be required if the direction of the spray of the jets could be more closely controlled. Such systems are also inefficient since the majority of time the spray from the jets is not effective directed to the wall of the vessel, but either up or down away from the surface to be cleaned.
Furthermore, since the distance from the center axis of the tank, where the jets are typically located, to the interior surface of the tank may be several feet, hard deposits cannot be adequately removed and thus the cleaning process is more a flushing process.
Systems for cleaning the interior surfaces of a tank encounter another serious problem in that the inside of the tank typically includes structural support plates extending laterally inwardly toward the axis of the tank. These plates represent surfaces which must be cleaned, and also present obstacles for the movement of the cleaning tool within the tank or vessel. As the cleaning tool is lowered into a tank from an access point at or near the top of the tank, the interior obstacles within the tank must be considered when directing a high velocity jet from a point off the axis of the tank.
Another proposed solution to the problem of the variations in interior geometries of tanks to be cleaned takes advantage of automation technology. The interior geometry of the tank, including inside diameter, height, and interior obstacles, are set into a programmable controller and the tool is then run into the tank. Unfortunately, such systems are highly complex, require a long setup time, and are very heavy and expensive. Further, the time and expense required to program and debug the programmable controller is often longer and greater than the total cost of a satisfactory cleaning job without such a controller. Since the system must be re-programmed for each tank geometry, such systems are currently not cost effective.
Thus, there remains a need for a system for cleaning the interior surfaces of tanks which is flexible, effective, and efficient. The present invention is directed to filling this long felt need in the art.